Artificial intervertebral disc assembly

ABSTRACT

An artificial intervertebral disc assembly including upper and lower end plates, an expandable bladder carried by the end plates and circumscribing a containment area therebetween. A biocompatible fluid is disposed within the containment area. A cylinder having a plurality of apertures therein projects from the lower plate into the containment area. A piston having a curvilinear upper surface is reciprocally moveable within the cylinder and projects from the open end thereof. A cylindrical projection depending from the upper plate bears against and mates with the upper surface of the piston so as to allow relative rotational movement of the end plates into and out of parallel alignment. By varying the size of the apertures in the cylinder and the elasticity of the bladder, the amount of axial load necessary to effect inward movement of the end plates can be varied.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication Ser. No. 60/547,035, filed Feb. 23, 2004.

BACKGROUND OF THE INVENTION

The present invention relates to an artificial intervertebral discassembly which is implanted in the vacated space after a diseased ordamaged intervertebral disc has been removed. It is important that suchassemblies remain parallel to and fixed against the end plates of thevertebral body when subjected to off-axis loading so as to mimic theloading characteristics of a natural disc and provide the patient withsubstantially the same range of motion as would be provided by a healthynatural disc. It is also important for the comfort of the patient if thestiffness of the disc assembly can be set depending on the size of thepatient. The assembly of the present invention obtains these objectives.

SUMMARY OF THE INVENTION

The disc assembly of the present invention is generally oval-shaped,provided in different sizes to cover the end plates of the vertebralbodies of differently sized patients and employs a novel bladder andpiston dampening assembly that axially biases the end plates of the discassembly outwardly against the end plates of the vertebral body, allowsfor variations in the stiffness of the assembly for differently sizedpatients as well as pivotal movement and shock absorption under a widerange of physiological loading conditions while continually maintainingeach of the assembly end plates in a parallel abutting relationship withthe adjacent end plate of the vertebral body. The end plates of the discassembly are each provided with outwardly projecting fixation featuresto decrease the chance of the implanted disc assembly migrating afterimplantation. The result of these features is a substantial improvementin artificial intervertebral disc replacement devices.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of the artificial intervertebral discassembly of the present invention.

FIG. 2 is a side view of the artificial intervertebral disc assembly ofthe present invention with the inflatable bladder being shown astransparent so as to show the positioning thereof and not to obscure theinterior of the disc assembly.

FIG. 3 is a perspective view of the upper end plate of the disc assemblyof the present invention.

FIG. 4 is a perspective view of the lower end plate of the disc assemblyof the present invention.

FIG. 5 is a side view of the artificial intervertebral disc assembly ofthe present invention.

FIG. 6 is a sectional view of the intervertebral disc assembly of thepresent invention taken along the line 6-6 in FIG. 5.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now in detail to the drawings, the artificial intervertebraldisc assembly 10 of the present invention comprises a pair of end plates12 and 14, a piston 16 and a bladder 18. End plates 12 and 14 arepreferably of an oval configuration with medial-lateral andanterior-poster dimensions in the same ratio as natural vertebral bodiesand are preferably constructed of a titanium alloy,Cobalt-Chromium-Molybdenum alloys and the like or a radiolucent materialsuch as polyetherketoneketone or polyetheretherketone (PEKK/PEEK),although they could be formed of other biologically acceptable inertmaterials that would provide the plates 12 and 14 with a rigidstructure.

The end plates 12 and 14 each define some form of fixation feature ontheir outer surfaces for engaging the end plates of the vertebral body(not shown) so as to decrease the chance the implant migration afterimplantation and to promote bone in-growth. In the embodiment of theinvention illustrated in the drawings, two or more dove-tailedprojections 20 are employed. Such dove-tailed projections engage matingsurfaces formed by the surgeon in the end plates of the vertebral body.Alternatively, a plurality of outwardly projecting spikes, ribs orridges or other convex surfaces that mate with the natural vertebral endplates could be employed to secure the end plates of assembly 10 inplace against the natural vertebral end plates. Long-term stabilizationof the implant may occur using a porous coating such as beads, anhydroxyappetite coating or plasma spray to promote bone in-growth.

The inwardly facing surface 12′ of the first or upper end plate 12, asshown in the drawings, defines a cylindrical projection 24 terminatingin a convex end face 24′ and a groove 26 extending about the perimeterof inner surface 12′ (see FIG. 3). The inner surface 14′ of the secondor lower end plate 14 defines a corresponding opposed groove 28extending about the perimeter thereof and a cylinder 30 projectingupwardly from the central portion of surface 14′. Cylinder 30 defines aplurality of radially extending channels 32 extending therethrough andis sized to receive reciprocating piston 16 therein. The upper endsurface of cylinder 30 is preferably beveled at 30′ as illustrated inFIG. 4. The upper surface 16′ of piston 16 is of a concave configurationso as to mate with the convex lower end face 24′ on the dependingprojection 24 on the first end plate 12 (see FIG. 6). The piston 16 isreciprocally moveable within cylinder 30 and is provided with a heightor axial length sufficient to project from cylinder 30 as shown in thedrawings and as will be further discussed.

Bladder 18 extends between end plates 12 and 14 and is held at its upperand lower end portions in perimeter grooves 26 and 28 so as tocircumscribe and enclose the space between the end plates 12 and 14disposed about piston 16 and cylinder 30 as shown in FIGS. 1 and 2.Bladder 18 is preferably formed of a flexible polymer material and issecured in place by a suitable epoxy adhesive or, alternatively, by amechanical connection such as a pair of locking rings (not shown)carried by the two end plates 12 and 14 or another suitable mechanicalattachment device.

The space disposed interiorly of the end plates 12 and 14 and bladder18, including the interior of cylinder 30 is filled with a biocompatiblefluid 31 such as saline or hydrogel. Piston 16 actually floats in thefluid 31 within cylinder 30. Accordingly, when an axially directedcompressive force is exerted on the end plates 12 or 14 or both, thepiston 16 is forced downwardly in the cylinder, causing fluid 31 in thearea between the piston and the bottom of the cylinder 30 to passoutwardly through the radial channels 32 in the cylinder wall, bulgingthe bladder 18 outwardly. Upon a relaxation of the axial pressureagainst the end plates, the resiliency in the bladder will cause fluidto move inwardly through channels 32, returning the assembly 10 to itspre-stressed configuration.

By sizing the channels 32, and varying the elasticity of bladder 18, thestiffness of the disc assembly, i.e., the axial force necessary toeffect the outward bulging or deformation of the bladder and thusrelative movement of the end plates can be regulated to accommodateindividuals of varying size and enable the disc assembly 10 to mimic thephysiological characteristics of a natural healthy intervertebral discand provide such individuals with a relatively full range of motion.

As seen in the drawings, the mating curvatures of the axially alignedupper surface 16′ of piston 16 and convex end face 24′ on projection 24and the positioning of the interface of those surfaces outside theconfines of the cylinder 30 allows relative rotational movement of theend plates about a plurality of axes while maintaining the abutment ofprojection 24 against piston 16. By providing rotation about a pluralityof axes, the two end plates 12 and 14 are allowed to move intonon-parallel orientations whereby the assembly 10 accommodates off-axialloading and mimics the distortion of a natural disc without adverselyaffecting the support function between the two end plates 12 and 14 thatis provided by the piston 16 and cylinder 30.

Various changes and modifications may be made in carrying out thepresent invention without departing from the spirit and scope thereof.Insofar as these changes and modifications are within the purview of theappended claims, they are to be considered as part of the presentinvention.

1. An artificial intervertebral disc assembly adapted to be implantedadjacent the end plates of the vertebral body following the removal of adiseased or damaged intervertebral disc, said assembly comprising: anupper end plate; a lower end plate, said plates each defining an innersurface and an outer vertebral body engaging surface; an expandablebladder carried by said plates and circumscribing a fluid containmentarea therebetween; a biocompatible fluid disposed within saidcontainment area between said end plates; a cylinder projecting fromsaid inner surface of said lower plate into said containment area andhaving an open upper end; a piston reciprocally moveable within saidcylinder and projecting from said open end thereof, said piston defininga curvilinear upper surface; a projection depending from said innersurface of said upper end plate into said containment area, saidprojection defining a curvilinear lower surface for mating with saidupper surface of said piston so as to allow relative rotational movementof said end plates and into and out of parallel alignment and aplurality of fluid flow apertures disposed in said cylinder below saidpiston.
 2. The assembly of claim 1 wherein said end plates areoval-shaped.
 3. The assembly of claim 1 wherein said outer surfaces onsaid discs define a plurality of spikes, ribs or ridges thereon forengaging the end plates of the vertebral body and preventing migrationof said assembly.
 4. The assembly of claim 1 wherein said outer surfacesof said discs define dove-tailed projections thereon for engaging matingsurfaces formed in the end plates of the vertebral body and preventingmigration of said assembly.
 5. The assembly of claim 1 wherein saidexpandable bladder is formed of a flexible polymer material having apredetermined measure of elasticity and wherein said apertures in saidcylinder collectively define a fluid flow area of predetermined size,said area cooperating with said measure of elasticity to define thedesired axial force necessary to effect relative inward movement of saidend plates.
 6. The assembly of claim 3 wherein said discs areoval-shaped.
 7. The assembly of claim 4 wherein said discs areoval-shaped.
 8. The assembly of claim 5 wherein said discs areoval-shaped.
 9. The assembly of claim 5 wherein said outer surfaces onsaid discs define a plurality of spikes, ribs or ridges thereon forengaging the end plates of the vertebral body and preventing migrationof said assembly.
 10. The assembly of claim 5 wherein said outersurfaces of said discs define dove-tailed projections thereon forengaging mating surfaces formed in the end plates of the vertebral bodyand preventing migration of said assembly.
 11. An artificialintervertebral disc assembly adapted to be implanted adjacent the endplates of the vertebral body following the removal of a diseased ordamaged intervertebral disc, said assembly comprising: an upper endplate; a lower end plate, said plates each defining an inner surface andan outer vertebral body engaging surface; an expandable bladder having apredetermined measure of elasticity carried by said plates andcircumscribing a fluid containment area therebetween; a biocompatiblefluid disposed within said containment area between said end plates; acylinder projecting from said inner surface of said lower plate intosaid containment area and having an open upper end; a pistonreciprocally moveable within said cylinder and projecting from said openend thereof, said piston defining a curvilinear upper surface; aprojection depending from said inner surface of said upper end plateinto said containment area, said projection defining a curvilinear lowersurface for mating with said upper surface of said piston so as to allowrelative rotational movement of said end plates and into and out ofparallel alignment and a plurality of fluid flow apertures ofpredetermined size disposed in said cylinder below said piston wherebyinward movement of said end plates will cause fluid in said cylinder topass outwardly through said apertures therein and expand said bladdersuch that variations in the collective area defined by said aperturesand the elasticity of said bladder will vary the amount of axial loadnecessary to effect such inward movement of said end plates.
 12. Theassembly of claim 11 wherein said end plates are oval-shaped.
 13. Theassembly of claim 11 wherein said outer surfaces on said discs define aplurality of spikes, ribs or ridges thereon for engaging the end platesof the vertebral body and preventing migration of said assembly.
 14. Theassembly of claim 11 wherein said outer surfaces of said discs definedove-tailed projections thereon for engaging mating surfaces formed inthe end plates of the vertebral body and preventing migration of saidassembly.
 15. An artificial intervertebral disc assembly adapted to beimplanted adjacent the end plates of the vertebral body following theremoval of a diseased or damaged intervertebral disc, said assemblycomprising: an upper end plate; a lower end plate, said plates eachdefining an inner surface and an outer vertebral body engaging surface;an expandable bladder carried by said plates and circumscribing a fluidcontainment area therebetween; a biocompatible fluid disposed withinsaid containment area between said end plates; a fluid flow pathdisposed between said end plates within said fluid containment area suchthat relative inward movement of said end plates effects fluid flowthrough said path so as to urge said bladder to expand outwardly suchthat variations in the size of said fluid flow path and the elasticityof said bladder will vary the amount of axial load necessary to effectsuch inward movement of said end plates.
 16. The assembly of claim 15wherein said end plates are oval-shaped.
 17. The assembly of claim 15wherein said outer surfaces on said discs define a plurality of spikes,ribs or ridges thereon for engaging the end plates of the vertebral bodyand preventing migration of said assembly.
 18. The assembly of claim 15wherein said outer surfaces of said discs define dove-tailed projectionsthereon for engaging mating surfaces formed in the end plates of thevertebral body and preventing migration of said assembly.
 19. A methodfor varying the axial force necessary to effect relative movement of theend plates in an artificial disc assembly to accommodate patients ofvarying size, said method comprising the steps of providing a fluid flowpath of predetermined size between the end plates of the assembly suchthat relative inward movement of said end plates effects fluid flowthrough said path and circumscribing an area between said end plates andabout said fluid flow path with a flexible bladder having apredetermined measure of elasticity.